The primary goal of this research is to measure the auditory system's frequency resolution in both normal listeners and listeners with a sensorineural hearing impairment. Frequency resolution will be determined by measuring the critical band in several dichotic listening tasks, including lateralization, localization, two-tone masking, and interaural frequency discrimination. In addition, masking by low-pass and high-pass noise will be used to explore the mechanisms underlying the lateralization of tone bursts that have very different frequencies in each ear. Monaural measurements of the critical band in impaired listeners will also be undertaken. Lateralization is measured by determining the minimum onset time difference between a tone burst in one ear and a burst in the other ear at which a listener correctly lateralizes the fused image toward the leading ear. The frequency difference between the tone burst in one ear and the burst in the other ear is varied parametrically. The critical band is taken as that frequency separation at which the lateralization threshold first begins to increase as the frequency separation is widened. Lateralization will also be measured by investigating the interactions between two pairs of tone bursts, each pair having its own onset time difference. The localization task will require the listener to identify the source of one tone burst coming from one loudspeaker in the presence of another tone burst, of a different frequency, coming from a second loudspeaker. In two-tone masking the detectability of a narrow-band noise will be measured in the presence of a pair of tones presented dichotically with a different frequency to each ear. Frequency discrimination will be measured by having the listener indicate which ear receives the lower of two frequencies.